1. Technical Field
The present invention relates to electronic assemblies. More particularly, the present invention relates to reworkably attaching integrated circuits and related devices to a substrate.
2. Description of the Prior Art
Semiconductor technology has advanced to the point where a designer has many options to choose from when mounting an integrated circuit die as part of an electronic assembly. For example, it is common practice to mount integrated circuit die directly to various substrates, such as packaging, circuit boards, flexible films, etc.
Circuit boards are presently the predominant means of forming electronic assemblies with integrated circuits and related components. One advantage of a circuit board is that, in addition to providing a substrate to which the integrated circuit may be mechanically attached, for example by an adhesive, circuit boards also provide electrical interconnection between the terminals of the integrated circuit and other electronic components in the electronic assembly. Various techniques are known for making electrical connections between the terminals of the integrated circuit and those of the circuit board, e.g. ball bonding, bumping, wire bonding, etc.
It is desirable, and often necessary, to remove defective or obsolete integrated circuits from such electronic assemblies, such that the assemblies may be reworked. In this way, an expensive circuit board containing many integrated circuits and other components may be repaired and reused, rather than being discarded. It is also desirable to remove integrated circuits from circuit boards for salvage purposes. For example, it is often useful to remove an expensive microprocessor from a circuit board for use elsewhere, or where the board is irreparably damaged.
Most adhesives that are commonly used to attach a die to a circuit board form a permanent bond between the integrated circuit and the circuit board. When these adhesives cure, an irreversible polymerization occurs. Once bonded to a circuit board in this way, a die may not be removed from the circuit board without destroying the die and damaging the board, along with other electronic components mounted on the board.
It is known that thermoplastic adhesives provide a bonding material that may be softened with heat and thereby reworked. However, the selection of softening and curing temperatures for such adhesives is quite limited. Thermoplastic adhesives that may be cured and softened at temperatures low enough to avoid damaging common circuit boards, such as are made of FR4 material having a low glass transition temperature ("Tg"), and neighboring electronic components during attachment or rework do not maintain a satisfactory bond under normal circuit operating conditions. Thus, a thermoplastic bond will fail when the bond is subjected to significant thermal stress, such as is typically found in a powered integrated circuit. Additionally, the likelihood of bond failure due to thermal stress is exacerbated by the temperature coefficient of expansion (TCE) mismatch between the silicon integrated circuit, about 2.3 cm/cm-.degree. C., and the circuit board material, about 17 cm/cm-.degree. C.
Thus, adhesives that provide a reworkable, stable bond for attaching an integrated circuit die to a common circuit board are not known. A process for reworkably attaching a die to a circuit board would provide more efficient electronic module production by eliminating the waste of otherwise repairable circuit boards and other components.